Process of coating carbonaceous material with silicon carbide



May 4, 1954 H. R. MONTGOMERY E-r AL 2,677,627

PRocEss oF coATING cARBoNAcEous MATERIAL WITH sILIcoN CARBIDE med oct.26. 1951 Inventors HAROLDY E Mo/vnsomf/ey Jw WALTER SzYMAszL-K ceorneyPatented May 4, 1954 UNITED STATES PATENT OFFICE PROCESS OF COATINGCARBONACEOUS MATERIAL WITH SILICON CARBIDE sachusettsApplication'October 26, 1951, Serial No. 253,302

Claims. (Cl. 117-406) The invention'relates to a process for coatingcarbonaceous material such as graphite with silicon carbide.

One object of the invention isto produce superior exhaust'nozzles forguided missiles. Another object is to produce superior rocket nozzles.

'Another object is to produce superior combustion chambers. Anotherobject isto produce superior "crucibles Anotherobjectof vtheinvention isto provide a process for forming silicon carbide in situ on graphite orother carbonaceous pieces with. simple equipment. Another object is toprovide a process for coating articles with silicon carbide whichvproducesrheavy and strongly Aadherent coatings. .Anotherobject-of theinvention is to provide a .thoroughly practical process for themanufacture of Venturi tubes and the like.

Other objects will be in part obvious or in part pointed outhereinafter. In the drawing the single figure is a vertical axialsectional view of Iwith Venturi bores that'will withstand 'intense heatfor a long enough time to be quite successful for use in such guidedmissiles and rockets.

We start with pieces of graphite because graphite can readily bemachined. For example the shapes I are made from slid'cylinders ofgraphite by boring them and then shaping the bores in a turret latheusing shaped tools. 'These A`shaped tools are made 'of soft steel andthis "greatlysimplies the problem of making the tools to Vthe 'exactshape desired; a problem which would be difficult using hard toolmaterial because the Venturi shape is not simple. Two tools are usedforshaping the bores, one for each end, and each tool cuts on each sidethereof thus to balance the cutting forces and then a third tool is used.to nish the smallest diameter oi the boreat the `limit of machining bythe other tools to make a bore smooth from end to end without vany toolmarks and of perfect Venturi shape.

The shapes `I Awhich are to be nozzles are set on graphite bars 2 whichare preferably triangular in cross section and are supported by a:circulargraphite plate 3 resting on graphite bars 4, which can besquare or rectangular in cross section, the bars 4 being placed acrossthe open top of an inner graphite crucible 5 lled almost full withsilica sand t. The graphite crucible 5 rests on graphite blocks 'i onthe bottom of an outer graphite crucible 8 having a graphite cover 9.

Through the cover 9, conveniently in the center thereof, is a bore l0.In a counterbore of the bore Iii is a graphite tube I! which acts as achimney. Extending into a dead end bore into the cover at an angle tothe vertical is a graphite tube i2 with two bores I3 and I4 and acut-out i5 at the inner end; this is a pyrometer tube and the reason forhaving two bores and a cut-out is to blow gas, suchas nitrogen, throughit from the outer end to get rid of fumes which would obscure the deadend of the bore in the cover the color of which is matched to the hotwire of the pyrometer in order to determine the temperature of theapparatus.

Outside of the cruclble 8 is coiled copper tubing i5 through whichvwater is owing and through the metalvof which an alternating current ofelectricity at high frequency is flowing. The coiled copper tubing it isthe primary of high frequency induction apparatus; the secondary is theCrucible 8 and the cover ii. Inside of the coil Iii-is a sheet ofasbestos Il coiled into a'cylinder and inside of the asbestos sheet Iland under the Crucible and over the cover 9 is comminuted zirconia i8which is very heavy and is a good insulator of heat and, where-cool ason the outside, a non-conductor of electricity. The bottom of the coilIt and the lower layer or zirconia rest upon fire clay bricks I9.

The process may now best be described by giving an illustrative example.

sample I For coating certain nozzles and cylinders with silicon carbidethe apparatus described was used the crucible having been thirty inchesin diameter and the other dimensions of the crucible 8 and of the otherparts of the apparatus having been in proportion about as shown in thedrawing. The nozzles in this case were 31/4 inches long and 2% inches indiameter with Venturi bores about the same shape as in the shapes l; butthese nozzles were smaller in proportion to the apparatus and there werethirteen of them. The apparatus having been assembled with the nozzleson bars 2, the induction machine (not shown) was started and deliveredelectric current at a frequency of 1000 cycles a second with a power or"240 kilowatts to the coil iii. The electromotive force was 365 volts.This input was maintained for two hours and twenty minutes whereupon theE. M F. was dropped to 285 volts and the power to 170 kw. and this inputwas maintained for one hour more and then the current was turned offcompletely and the apparatus was allowed to cool. The nozzles were foundto be coated with silicon carbide inside and outside to a depth of about.020" to .030. The temperature inside of the Crucible 3 for the lasthour was approximately 2350 C.

rllhe process is believed to work as follows: The silica sand 6 melts atabout 1700 C. and boils at 2230 C. Therefore when the Crucible 5 reachedabout 2240" C. the liquid silica boiled violently. At first the gasespassed off through the chimney II but later that became plugged withcondensed silica and then an appreciable pressure built up within theCrucible 8; sometimes the cover 9 with the heavy zirconia I 3 thereonhas been lifted and when that happened the power was cut down for awhile. In the particular run described in Example I, however, the Cover9 did not rise.

The silica vapor in the Crucible 8 on striking the pieces I reacts withthe graphite (carbon) to form silicon carbide which is thus integralwith the pieces i. Of course the inside of the Crucible 8 and the underside of the cover 9 are also coated with silicon carbide but that is nota detriment. The irst time the apparatus is used a large amount ofsilica sand is used up coating the inside of the Crucible 8; thereafterless silica sand is used up because the coating process proceeds moreslowly after the initial stages as the graphite does not have such readyaccess to the silica vapor. But the silicon carbide layer is somewhatporous so the reaction and the coating and lining of the pieces I takesv,

less time than in the irst operation. The operation described in ExampleI was done with apparatus which had been used before. As an incident ofthe process the plate 3 and also the crucible 5, the bars 2 and 4 andthe blocks l are likewise coated with silicon carbide.

Our process is much more effective than the process of coating graphiteand the like by heating it in the presence of silicon. Silicon carbidedissociates at 2500" C. so therefore in any process for coating anythingwith silicon carbide the temperature has to be kept below that ligure,but silicon does not boil until it reaches 2600 C. so the process ofcoa-ting graphite with silicon carbide by heating it in the presence ofsilicon is dependent upon the presence of a minor proportion of siliconvapor in the atmosphere and the process is slow and it is diliicult toform a thick coating using the silicon process. But in our process theatmosphere in the Crucible 8 is heavily charged with silica vapor SiO2,the coating of silicon carbide is relatively quickly formed and thickstrong coatings can be made.

The reason why we use a plate 3 to cover the Crucible 5 instead oflaying the bars 2 across an open top Crucible 5 with the pieces i on thebars 2 is that when the silica boils a blast of vapor is formed whichwill corrode the pieces I, that is to say eat them away whenever thevapor impinges thereon. In other words the plate 3 is a baille to breakup the flow and to cause turbulence and to shield the pieces I from afast moving stream of gaseous silica. At all events the setupillustrated is effective While removal of the plate 3 frequently spoilsthe pieces I. The baille plate 3 thus cuts down the velocity of thesilica gas and any other arrangement to cut down the velocity of the gasat the locus of the pieces I Could be used. This feature may be referredto as protecting the pieces from direct impact of the silica gas. Wewill now give another example of the process of the invention.

Example II Using a graphite Crucible 8 of the same size as alreadydescribed and other parts in proportion about as shown in the drawing wecoated twelve graphite pieces to form nozzles. The pieces were 3% incheslong and 21/8 inches in diameter and the bores had about the sameVenturi shape as the bores in the pieces I in the drawing. For one hourand a half the coil I 6 was energized with electric current at 315volts, 230 kilowatts and a frequency of 1000 cycles per second, then forone hour at 315 volts, 240 kilowatts and the same frequency, then forone hour at 285 volts, 170 kilowatts and the same frequency. Theapparatus had been used before so the inside of the Crucible B and theunder side of the cover 9 and the Crucible 5 and plate 3 were coatedwith silicon Carbide. The temperature of 2350 C. was reached and heldfor the last hour. The pieces were well Coated, the coating was about.02 to .030" thick and absolutely integral with the underlying graphite.The outside diameter grew only .004" to .006 (.002" to .003" on theradius) showing that a good deal of the graphite was Converted tosilicon carbide. (The Venturi bore was coated with silicon Carbide .020"to .030 thick and was everywhere reduced in diameter by .004 to .006".)Thus it is not a case of merely forming a coating on a piece ofgraphite, an outer layer and also an inner layer of graphite isconverted to silicon carbide while more silicon Carbide forms thereover.The same result was also achieved in Example I.

Example III Using similar apparatus as in Examples I and II with an oldcrucible 8 well coated with silicon Carbide on the inside and an oldcover 9 well coated with silicon carbide on the under side but with anew plate 3 and Crucible 5 (the blocks 1 were omitted in this run inorder t-o leave enough space for the pieces) the coil I 6 was energizedat 320 volts, 220 kilowatts, 1000 Cycles to coat four graphite pieces tomake nozzles, 8 inches in diameter, 12 inches long with Venturi bores,and to coat four hollow Cylinders to make chambers, 8 inches outsidediameter, 71/2 inches inside diameter and 10 inches long. The aboveinput was maintained for one hour then the load increased to 240kilowatts, voltage same. This input was maintained for another hourwhereupon, with the same voltage the load increased to 250 kilowatts. Atthis time, namely at the end of two hours, the chimney II was found tobe plugged. After another half hour the voltage was dropped to 250 andthe load became 170 kilowatts. The temperature had now reached about2350 C. The latter input, 260 volts, 170 kilowatts was maintained foranother hour holding the temperature inside the Crucible 8 at 2350o C.or thereabouts. Then the current was cut 01T and `the apparatus wasal1owedto'cool.

The` pieces were found to be coated every-whereby a coating 3.020" to.030 deep but ha'd grown (outside and inside) only .002 -to '.003 on theradius. Of course this growth makes the bore smaller by the amountstated.

The chimney Il (which was 24 inches high) took on a coating of siliconcar-bide in its bore adjacent the lower endl but frequently becameplugged with silica about half way up. There was always some plugging ofthe chimney with silica .but this plugging didv not in every runcompletely block the chimney although in many runs it did. This pluggingwith silica rather conclusively shows that the atmosphere in thecrucible was silica and not silicon. However we believe that the silicain vapor phase may have included some silicon monoxide, SiO, as well asthe vapor phase silicon dioxide, SiOz. The oxygen lost would combinewith carbon to form carbon monoxide. All air originally in the crucible8 had been driven out of the chimney Il long before the pluggingoccurred. The same chimney tube ll was, however, used over and overagain since the silica plugging it was removed with no great diic'ultywith iron rods.

The Venturi bore may, in some cases, be a bore comprising a pair offrusto-cones on a single axis with the small ends towards each other butmore often the bore is a non-symmetrical negative hyperboloid. Anhyperboloid is a volume clened by rotating an hyperbole. around an axis.The hyperboloid is negative when its surface is concavo-convex. The boreis non-symmetrical when one mouth is larger than the other one. If thepieces are made of amorphous carbon hard cutting tools are required.Such tools are more difcult to shape to curves such as hyperbolas andtherefore it is an advantage to cut the pieces out of graphite which canbe cut with soft steel since the soft steel tools can be more easilyshaped. The cutting tools are usually made to approximate shape in alathe and then iinished by hand to accurate hyperbole. shape using filesand abrasive cloth.

A great advantage of our process is that there L is so little growth(and what growth there is is evenly distributed) so nozzles can be madewithin tolerances of plus or minus one thousandth of an inch everywhere.Allowance is, of course, made for such growth as is obtained.

For the manufacture of very large nozzles the process of the inventioncan also be used for coating sectors of cylindrical pieces with Venturibores so that when the sectors are assembled a complete nozzle will beformed. In one case the shape consisted of twenty sectors which werecoated with silicon carbide and eventually assembled in a steel shellwith refractory cement.

Within the limits of this invention `any carbonaceous material can becoated which will remain in the solid phase at 2230 C. This includesamorphous carbon and graphite and pieces of other material mixedtherewith. The range of top temperatures for carrying out this inventionis from the boiling point of silica (which we believe is close to 2230oC.) to the incipient decomposition o1 silicon carbide (we believe itstarts to decompose at about 2450 C.

although decomposition is not rapid until about 2500 C.). Furthermore atabove 2450 C. the boiling of the silica appears to be too violent forthe best results. The temperature should be maintained -between theselimits for at least one half hour to produce an adequate coating.

We have found it is really-immaterial whether the chimney H becomesplugged or not and in any event the pressure inthe crucible 8 neverrises much above one pound per square inch gauge pressure and frequentlyis at atmospheric pressure.

Graphitehas great thermal shock resistance and can be easily machinedinto desired shapes. However it oxidizes readily at high temperatures.Silicon carbide is very refractory and not easily oxidized even at thetemperatures of the jets' in rockets and other missiles. It is resistantto the erosive effect of the jets, the gases of which sometimes reach avelocity of 4000 feet per second, while graphite is not resistant tosuch erosive effect. And silicon carbide is diicult to shape and almostimpossible to machine. Hence by coating graphite with silicon carbide,we utilize the good properties of each material without detriment fromthe lbad properties of either.

Features of the invention are that the pieces to be coated are spacedfrom the comminuted silica (silica sand) and are not buried in anything,i. e. they are surrounded by air in the first place and during theprocess they are surrounded by gaseous silica with some carbon monoxidemixed therewith instead of being buried in the sand. Another feature isthat 4the heating is done by electric induction which makes possibleexcellent control without which control the results are sporadic anduncertain. Furthermore, induction heating makes possible uniform heatingwhich is highly desirable.

It will thus be seen that there has been provided by this invention aprocess for coating carbonaceous material (especially graphite) withsilicon carbide in accordance with which the various objectshereinbefore set forth together with many thoroughly practicaladvantages are successfully achieved. As many possible embodiments mightbe made of the mechanical features of the above invention and as manychanges might be made in the embodiments above set forth it is to beunderstood that all matter hereinbefore set forth or shown in thedrawing is to be interpreted as illustrative and not in a limitingsense.

We claim:

1. Process of coating a carbon body with silicon carbide comprisingplacing said carbon body in a closed carbon container containingcomminuted silica spaced from said body, heating the container to theboiling point of the silica while protecting the body from direct impactof the gaseous silica formed and keeping the temperature below thetemperature of incipient decomposition of silicon carbide andmaintaining the temperature of the container between the boiling pointof silica and the temperature of incipient decomposition oi siliconcarbide for at least one half hour, the coating being formed by thereaction of silica gas with the carbon material.

2. Process according to claim 1 in which the body is a body of graphite.

3. Process according to claim 2 in which the container is made ofgraphite.

4. Process according to claim l in which the container is made ofgraphite.

5. Process for the manufacture of articles coated with silicon carbidecomprising machining a piece of graphite to the required shape, placingit in a closed carbon container with a quantity of comminuted silicatherein but spaced from the comminuted silica, heating the container tothe boiling point of the silica while protecting the `piece from directimpact of the gaseous silica formed and keeping the temperature belowthe temperature of incipient decomposition of silicon carbide andmaintaining the temperature oi the container between the boiling pointof silica and the temperature of incipient decomposition of siliconcarbide for at least one half hour, the coating being formed by thereaction of silica gas with the graphite.

6. Process according to claim 5 container is made of graphite.

7. Process according to claim 1 in which the heating is done by electricinduction whereby to secure substantially uniform heating of the silicaand of the carbon bcdy.

8. Process according to claim 7 in which the carbon body is a body ofgraphite.

in which the 9. Process according to claim 8 in which the container ismade of graphite.

1G. Process according to claim 1 in which the heating is done byelectric induction whereby to secure substantially uniform heating ofthe silica and of the carbon body and in which the container is made ofgraphite.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,658,647 Miller Feb. 7, 1928 2,431,326 Heyroth Nov. 25, 19472,439,983 Morgan et al Apr. 20, 1948 2,463,791 Morgan Mar. 8, 1949

1. PROCESS OF COATING A CARBON BODY WITH SILICON CARBIED COMPRISINGPLACING SAID CARBON BODY IN A CLOSED CARBON CONTAINER CONTAININGCOMMINUTED SILICA SPACED FROM SAID BODY, HEATING THE CONTAINER TO THEBOILING POINT OF THE SILICA WHILE PROTECTING THE BODY FROM DIRECT IMPACTOF THE GASEOUS SILICA FORMED AND KEEPING THE TEMPERATURE BELOW THETEMPERATURE OF INCIPIENT DECOMPOSITION OF SILICON CARBIDE ANDMAINTAINING THE TEMPERATURE OF THE CONTAINER BETWEEN THE BOILING POINTOF SILICA AND THE TEMPERAATURE OF INCIPIENT DECOMPOSITION OF SILICONCARBIDE FOR AT LEAST ONE HALF HOUR, THE COATING BEING FORMED BY THEREACTION OF SILICA GAS WITH THE CARBON MATERIAL.